Infusoria is also easy to cultivate..i have a pond with water weed and some plants..its not to clear and the water is greenish which means there is infusoria in the pond..so almost every pond has infusoria. I'm not saying to go taking water from random ponds and putting it into your tank ...but you can try this also, put some lettuce leaves in some warm ( not hot water) in a bucket and place it outside or somewhere where there is sunlight. Leave for a couple of days and when u see it turning greenish that indicates there is infusoria in there. Now this would only apply if the climate where you at it warm..if not you dont have to try this.